A PLATINUM SURFACE AT HIGH TEMPERATURES. 507 



The usual definition of the emissivity of a body, and the one I have adhered to 

 throughout, may be expressed as follows : The emissivity is numerically equal to the 

 number of gramme-degrees, or therms, dissipated per second, per square centimetre of 

 surface, per degree above the enclosure. The " true emissivity " at any temperature, 

 t, might be defined as the slope at the temperature t of the tangent to the curve 

 representing the total heat dissipated plotted in terms of the temperature. In other 

 words, if h is the loss of heat per square centimetre per second at any temperature, t , 

 the " true emissivity " at this temperature is dh/dt. 



The Curves II. and III. (fig. 7), if plotted in terms of the true emissivity, would almost 

 coincide, showing that in this case, at any rate, the value obtained depends to a very 

 limited extent on the temperature of the enclosure. 



A comparison of Curves I. and V. will show the effect of using a wire '6 millim. 

 instead of 1*12 millims. in diameter. 



Finally, in fig. 7, the points marked AA' refer to a platinum wire, 2'005 millims. 

 in diameter, that is to say, nearly twice the diameter used in all previous experi- 

 ments. The wire was fused in the gun-metal enclosure, and the results must therefore 

 be compared with the point B, obtained under identical conditions, but with a wire 

 1'12 millims. in diameter. 



It will be seen that the decrease in emissivity due to the use of the larger wire is, 

 at this temperature, less than 7 per cent. 



The emissivity at the points of fusion of palladium and platinum were deduced 

 from the readings of instruments of the Weston type, which were afterwards cali- 

 brated by aid of the potentiometer. The wire was raised to within about 100 or 

 200 of the melting point. The current was then increased by equal amounts of 

 \ or per cent., and current and electromotive force readings were taken at each 

 successive rise. The current was noted at the instant the wire fused, the correspond- 

 ing electromotive force being easily found by extrapolating the last few readings. 

 0-ver 100 amperes were needed to fuse the 2 millims. wire, and for accurate results to 

 be obtained it was necessary for this large current to remain constant to within a 

 tenth of a per cent. This experiment, as many of those previously recorded, was only 

 rendered possible by the exceptional facilities available at the Davy-Faraday Labora- 

 tory. 



A large proportion of the work done on the subject of emissivity refers to the 

 cooling of comparatively large bodies. This method was used by DULONG and PETIT, 

 J. C. NICHOL,* McFARLANE.t C. F. BRUSH,! Dr. J. T. BOTTOMLEY, and many others. 

 These results cannot serve as points of comparison. Added to the effect of the abso- 

 lute size of the bodies used, the question of the relative surfaces of the enclosure and 

 radiator comes here into play. Whenever the ratio of these two surfaces is not very 



* ' Proc. Roy. Soc.,' Edin., 1869, vol. 7, p. 206. 

 f ' Proc. Roy. Soc.,' 1872, vol. 15, p. 90. 

 J ' Phil. Mag.,' vol. 45, p. 31, Jan., 1898. 

 3 T 2 



